1. Field of the Invention
The present disclosure relates to sensors, and more particularly to infrared sensors such as used in infrared imaging systems and the like.
2. Description of Related Art
InGaAs, or indium gallium arsenide, is an alloy of gallium arsenide and indium arsenide. In a more general sense, it belongs to the InGaAsP quaternary system that consists of alloys of indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP), and gallium phosphide (GaP).
To a large extent, the electrical and optical properties of a semiconductor depend on its energy bandgap and whether the bandgap is “direct” or “indirect.” The energy bandgaps of the four binary members of the InGaAsP quaternary system range from 0.33 eV (InAs) to 2.25 eV (GaP), with InP (1.29 eV) and GaAs (1.43 eV) falling in between.
A semiconductor will only detect light with photon energy larger than the bandgap, or put another way, with a wavelength shorter than the cutoff wavelength associated with the bandgap. This “long wavelength cutoff” works out to 3.75 μm for InAs and 0.55 μm for GaP with InP at 0.96 μm and GaAs at 0.87 μm.
By mixing two or more of the binary compounds, the properties of the resulting ternary and quaternary semiconductors can be tuned to intermediate values. The challenge is that not only does the energy bandgap depend on the alloy composition, so also does the resulting lattice constant. For the four binary members of the InGaAsP quaternary system, the lattice constants range from 5.4505 Å (GaP) to 6.0585 Å (InAs) with GaAs at 5.6534 Å and InP at 5.8688 Å.
The InAs/GaAs alloy is referred to as InxGa1-xAs where x is the proportion of InAs and 1- x is the proportion of GaAs. One challenge is that while it is possible to make thin films of InxGa1-xAs by a number of techniques, a substrate is required to hold up the thin film. If the thin film and the substrate do not have the same lattice constant, then the properties of the thin film can be severely degraded.
Traditionally, the most convenient substrate for InxGa1-xAs is InP. High quality InP substrates are traditionally used with diameters as large as 100 mm. InxGa1—xAs with 53% InAs is often called “standard InGaAs” without bothering to note the values of “x” or “1−x” because it has the same lattice constant as InP and therefore the combination leads to very high quality thin films.
Standard InGaAs has a long wavelength cutoff of 1.68 μm. It is sensitive to the wavelengths of light that suffer the least signal dispersion and transmit furthest down a glass fiber (1.3 μm and 1.55 μm), and can detect “eye-safe” lasers (e.g., wavelengths longer than 1.4 μm). It is also considered the optimum wavelength band for detecting the natural glow of the night sky.
While standard InGaAs has a long wavelength cutoff of 1.68 μm, many applications require the detection of light with longer wavelengths. An important example is the ability to measure moisture content in agricultural products by measuring water absorption at 1.9 μm.
Another example is “LIDAR” (light detection and ranging), used in aircraft, for example, to detect clear air turbulence. LIDAR systems often use lasers that emit light with a wavelength of 2.05 μm. InxGa1-xAs with a longer cutoff is called “extended wavelength InGaAs.”
Unfortunately, obtaining the longer cutoff is not as simple as adding more InAs to the alloy. Doing so would increase the lattice constant of the thin film, which causes a mismatch with the substrate, and this reduces the quality of the thin film. Considerable effort has been put into learning to grow high quality extended wavelength InGaAs.
Such methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for improved sensors. The present disclosure provides a solution for this need.